Many forms of cognitive disease represent a steadily growing medical and social problem of our aging societies around the world. The prevalence of cognitive disease, for example dementia in North America, is approximately 6 to 10% of the population, with Alzheimer's disease accounting for a substantial portion of these cases. It is also recognized that many other neurological and psychiatric disorders may display symptoms of cognitive impairment. Some believe the main pathological features of Alzheimer's disease may relate to intraneuronal neurofibrillary tangles, formation of amyloid beta plaques and/or neurodegeneration of mainly cholinergic and, in later stages, also serotonergic, noradrenergic, and other neurons, resulting in deficiencies of acetylcholine and other neurotransmitters. Some theories suggest that the gradual development of an acetylcholine signaling deficiency may be responsible for the early clinical manifestations of cognitive disease. Consequently, some believe that compounds that improve cholinergic functioning, such as acetylcholine esterase inhibitors may ameliorate the cognitive deficits in patients with cognitive disease. The most widely used acetylcholine esterase inhibitor is donepezil hydrochloride (Aricept®). In addition to Alzheimer's disease, cholinergic deficits (reductions in neurotransmitter and/or receptor levels) are observed in other disorders where there are cognitive deficits, such as schizophrenia, major depressive disorder, and Parkinson's disease.
Nicotinic acetylcholine receptors (nAChR) form a large family of ion channels which are activated by the neurotransmitter acetylcholine which is produced in the body (Galzi and Changeux, Neuropharmacol. 1995, 34, 563-582). A functional nAChR consists of five subunits which may be different (certain combinations of α1-9 and (β1-4,γ,δ,ε subunits) or identical (α7-9). This leads to the formation of a diversity of subtypes which differ in the distribution in the muscles, the nervous system and other organs (McGehee and Role, Annu. Rev. Physiol. 1995, 57, 521-546). Activation of nAChR leads to influx of cations into the cell and to stimulation of nerve cells or muscle cells. Selective activation of individual nAChR subtypes restricts this stimulation to the cell types which have a corresponding nAChR subtype and is thus able to avoid unwanted side effects such as, for example, stimulation of nAChRs in the muscles. Clinical experiments with nicotine and experiments in various animal models indicate that central nicotinic acetylcholine receptors are involved in learning and memory processes (e.g. Rezvani and Levin, Biol. Psychiatry 2001, 49, 258-267). Nicotinic acetylcholine receptors of the alpha7 subtype (α7 nAChR) have a particularly high concentration in regions of the brain which are important for learning and memory, such as the hippocampus and the cerebral cortex (Seguela et al., J. Neurosci. 1993, 13, 596-604). The α7 nAChR has a particularly high permeability for calcium ions, modulates neurotransmission, influences the growth of axons and, in this way, modulates neuronal plasticity (Broide and Leslie, Mol. Neurobiol. 1999, 20, 1-16).
WO 2003/055878 describes a variety of agonists of the α7 nAChR said to be useful for improving cognition. WO 2003/055878 suggests that certain agonists of the α7 nAChR are useful for improving perception, concentration, learning or memory, especially after cognitive impairments like those occurring for example in situations/diseases/syndromes such as mild cognitive impairment, age-associated learning and memory impairments, age-associated memory loss, Alzheimer's disease, schizophrenia and certain other cognitive disorders.